Why the big flash units? SB18's will do the trick. I learned this from John Shaw. As a matter of fact he claims this is the only field flash he considers for macro.

If you havent read his book 'Closeup's In Nature' I highly recommend that you give it a go.

As I mention, I use a SB-18 for background. I don't want to use them for close in flashes because 1> I want adjustable power instead of gels and 2> I want to use them for slaving in other non macro shots as well.